Heat exchangers are known, which direct water in a pipe through a flue.
U.S. Pat. No. 4,122,801 of Burns describes a heat exchanger which first encircles water pipes in 2 circular rings around an exhaust flue, but then directs the water into coils within the exhaust flue. The external rings appear to be for positioning the water flow headers equally around the flue, not to preheat the water.
U.S. Pat. No. 4,211,187 of Farris discloses an energy conservation system for heaters that uses a heat exchanger in a furnace chamber or duct.
U.S. Pat. No. 4,136,731 of DeBoer discloses a heat transfer apparatus for supplementing a building heating and cooling system, using a heat exchanger in a furnace flue. DeBoer suggests an external heat exchanger, but provides no enabling details thereof.
Japanese patent no. JA0031286 of Satou discloses a heat transfer pipe for high temperature gases. It has a shell with multiple, non-axial connected heat exchangers.
U.S. Pat. No. 6,068,047 of Buchhave describes a heat exchanger for a sludge containment structure made of two rigid half shells, wherein sludge flows in the annular space enclosed by the shell halves and an inner sleeve in a spiral path external the spiral tubing with heating fluid flowing therethrough.
U.S. Pat. No. 4,484,564 of Erickson disclose a water heater utilizing exhaust gases from furnaces or stoves, but the recovery is through a coil inside an exhaust flue.
U.S. Pat. No. 1,990,056 of Van Daam describes passing water through a spherical corrugated chamber.
U.S. Pat. No. 4,251,028 of Nicolai discloses a preheater with an internal wall parallel to an external side wall with a water tight seal with pressure restraining capability. It does not disclose a coil of hollow heat conductive tubing enclosed within a substantially dry cavity.
U.S. Pat. No. 3,896,992 of Borovina and No. 2,521,462 of Kinzelmann both disclose water heaters that pass water through a spiral coil within an exhaust flue.
Canadian patent no. CA1271380 of Hampden describes a heat exchanger for air, not fluids, which is heated in a replaceable flue section. Hampden uses a blower to draw air through fins in an annular space.
U.S. Pat. No. 4,037,567 of Torres proposes an exhaust flue over the water heater having a spiral coil for heating water therein.
U.S. Pat. No. 4,120,267 of Wood describe a tube and plate heat exchanger with water heating coils inside a chamber, such as a gas heat duct or flue.
German patent no. DE 3340281A of Grabietz describes a flue within a jacket wherein coiled water tubes wrap around the inner flue pipe. However, the space between the tubes positioned over the inner flue and within the outer jacket are filled with solid, cast molten metal, instead of fibrous insulation.
Soviet Union patent no. SU0779719 of Ukrorgtekhstroi discloses a heat exchanger with inner fins, not coils, which has heat resistant end caps enclosing a flue pipe. The fins are spiraled, but they are not hollow and do not carry fluid therein. Ukrorgtekhstroi does not contain tubing wrapped around a flue pipe.
Furthermore, U.S. Pat. No. 4,401,261 of Brown also discloses directing water coils inside of flues.